A sheet-formed article made of general transparent plastic or *a glass exhibits high transparency to light entering at any incident angle and is used as windows and various other optical materials. The sheet can be imparted with a function to scatter the light applied to the sheet by forming irregularity on the surface of the above transparent glass or uniformly dispersing fine particles of several microns order in a plastic sheet, and these sheets are being used also as a light-scattering plate. It is known that the viewing angle of a liquid crystal display device can be improved to attain a wide viewing angle by placing the light scattering plate in front of a polarizing plate placed in front of a liquid crystal display device.
However, these light-scattering plates randomly scatter an incident light entered from any direction and are free from function to selectively scatter an incident light entered at a specific incident angle.
On the other hand, a transparent formed article having a light transmittance dependent upon an incident angle to vary the light-scattering property varying with angle can be utilized e.g. as a viewing angle control plate which can be seen from an angle and is difficult to see from another direction.
A diffraction-type optical element having light diffraction effect and produced by forming a grate-like unevenness on the surface of a transparent substrate made of glass, plastic, etc., is already known as an optical element having the above characteristics. However, the production of such optical element necessitates a highly elaborate manufacturing process, etc., and, although it is effective in certain particular uses, it has a problem of low productivity and high cost in a use necessitating an optical element having a large area.
The specification of the Japanese Patent TOKKAISHO 62-65957 describes a photochromic glass which is highly transparent when seen from the side of an observer and shields the light entering from a direction other than the observer. The photochromic glass can be produced by forming a fine steric pattern formed by curing a photosensitive resin and placing the pattern between a pair of glass plates, and a highly elaborate manufacturing process is necessary for forming the steric pattern. Furthermore, the manufacturing process comprises a number of troublesome steps and the glass is colored by the irradiation of light.
Light-control plates scattering only the light of a specific incidence angle are described in the specifications of the Japanese Patent TOKKOHEI 7-58361, TOKKAISHO 64-40902 and TOKKAISHO 64-77001. The light-control plate has a multilayer structure composed of a photo-setting resin having two microscopic regions arranged parallelly and periodically in a definite direction, and is produced by using two or more kinds of polymerizable monomers and/or oligomers giving cured resins having different refractive indices. It is necessary in the above case to optimize the polymerizability, refractive index difference, etc., of these monomers, etc. Since the photo-set layer has poor resistance to external stress, the layer is supposed to be used mainly in a structure sandwiched between a pair of plastic films, etc., or laminated on a transparent substrate such as a glass plate. There is a problem in productivity of the plate owing to the troublesome manufacturing process of the plate necessitating many steps.
The specification of the Japanese Patent TOKKAIHEI 9-166702 (Laid-open date: Jun. 24, 1997) discloses a process for the production of a light-control film having a light transmittance dependent on an incident angle by immersing a polymer film in a non-solvent of the polymer and/or a solution including a surfactant and winding on a roll. This production process is complicate because it necessitates the immersion of a film in a solvent, etc., the bending of the immersed film with a roll to generate cracks (called as crazes in the specification) on the surface of the film and the removal of the solvent, etc., from the film. The crazes are generated exclusively on the surface of the film degenerated by the solvent, etc., and, accordingly, the optical properties of the film are determined by the surface morphology of the film. However, the control of the surface morphology seems to be difficult because a solvent, etc., is used in the production. Furthermore, it is difficult to narrow the pitch of crazes and, accordingly, the production of e.g. a film having high haze by front view and low haze by oblique views is supposed to be difficult. Other possible problems are the liability of the curling of the film and the considerable change in the optical properties in the case of the application of the hard-coating treatment, etc., to the crazed surface owing to the surface properties determining the optical properties.